Modern heating, ventilating, and air conditioning (HVAC) systems may be computer controlled in order to maximize comfort and minimize cost and energy consumption. Computer control permits the temperature of and air flow rate to different areas within a building to be tailored to the specific needs required by the different building areas and the changes in those needs in accordance with a daily and weekly schedule. A building management system designed for this purpose typically has a central computer which is controlled by an operator, a plurality of remotely controlled HVAC system components, such as dampers and fans, and a plurality of remote sensors, such as temperature, humidity, and air flow sensors.
The present invention is directed to an improved sensor for detecting a signal representing the flow rate of air at a particular location within a duct in the HVAC system.
The prior art has shown various sensors for accomplishing this function. One of the first sensors utilized the Pitot tube system, in which a small, elongated tube extends into the air stream of a duct with its open end pointing upstream. Its opposite end is closed by a static pressure sensor. The Pitot tube system measures a static pressure within the tube which is an increasing, continuous function of the air flow rate within the duct.
Because such a system detects stair pressure it describe air flow rate in terms of a pressure or pressure differential, commonly in units of inches of water. Such prior art systems develop electrical signals which are a function of the static pressure. In order for a newer sensor to be compatible and therefore replace an earlier sensor, the subsequent sensors also develop signals which are based upon the units of inches of water.
In computer control systems, information is stored in the computer for each remote sensor and its local conditions. For example, the stored data would include both the cross-sectional areas of each duct where a flow rate detector is located and an algorithm for converting the signal from the flow rate detector to flow rate so that the air volume in cubic feet of air per minute flowing through a duct may be calculated and utilized in the control of the environment.
A more recently developed prior art system for detecting air flow rate in an HVAC system uses a small tube having its open, inlet end facing-upstream and an open outlet end located downstream of the inlet end and facing downstream. This tube contains a pair of electrical conductors, such as thermistors, one upstream of the other and located within the tube. One of these conductors is heated by applying a constant electrical power to it and the other conductor is not heated. Electronic circuitry attached to these conductors measures the impedance of each conductor and therefore detects a signal representing the temperature differential between the two conductors.
This prior art "hot wire" system operates on the theory that a small portion of the air stream is diverted through the tube and the flowing, diverted air stream causes a pressure differential as a result of the fluid flow resistance through the tube. The higher the fluid flow velocity through the tube the more heat is carried away from the heated wire in the flowing air, and therefore the more it is cooled toward the temperature of the unheated conductor. The more the heated wire is cooled the less is the temperature differential which is measured between the two conductors. Consequently, the temperature differential, which is measured by the electronic circuitry, is a function of the air flow rate in the duct and of the pressure differential between the ends of the tube.
While such a system works reasonably well at relatively smaller flow velocities, when the flow velocity becomes substantial, the heated wire is cooled so much that accuracy is lost and a low measurement resolution is the result. Consequently, such prior art systems have a relatively narrow range of accuracy. For example, they typically have a range of accuracy of 0.03-2.0 inches of water. Variations in the temperature of the air also have a substantial affect on the accuracy of the device.
However, in modern HVAC systems it is desirable to have the range extending from 0.01 inches of water to 4 inches of water.
There is, therefore, a need for an air flow sensor which can operate over the entire range of 0.01 to 4.0 inches of water and over a temperature range of 40.degree. F. to 90.degree. F. while providing resolution accuracy better than 0.01 inch of water.